The circuit structure of an active-matrix OLED display, in which a plurality of pixels are arranged in rows and columns, is widely known. Each pixel includes two thin film transistors (TFTs), i.e., an addressing (or switching) transistor and a driving (or power) transistor, a storage capacitor, and an OLED device.
As is well known, in the conventional active-matrix OLED panel circuit, a scan line (row line) is selected, a video signal loaded in a data line (column line) is input to the driving transistor via the addressing transistor to control the current through the OLED device. The video signal is stored in the storage capacitor for the duration of one frame.
TFTs used in active-matrix OLED display panels are formed by use of amorphous silicon, polysilicon, or cadmium selenide (CdSe) through manufacturing processes such as photolithography or evaporation by use of a shadow mask technology. Threshold voltage variation in such a TFT, which may be caused by variations in the manufacturing process, leads to current non-uniformities between pixels and non-uniform brightness. These problems are not significant in small-screen applications, such as flat-panel displays in watches, telephones, laptop computers, pagers, mobile phones, calculators, and the like. However, in a large-screen display application, such as a flat-panel television, the display undergoes more serious threshold non-uniformities, and the quality of the display, such as brightness uniformity, is noticeably degraded.
The light output depends on several factors—(1) the uniformity of the power transistors at the time of manufacture, (2) the uniformity of the power transistors as they age, and (3) the stability of the medium itself that is being driven. Therefore, there is a technical challenge in ensuring that the power transistors that drive the OLEDs are uniform and, secondly, if they are not uniform, there is a technical challenge in correcting the non-uniformity. A need exists for a way to compensate the active-matrix power transistors so that they are uniform and, thus, the brightness of the active-matrix OLED display is uniform from pixel-to-pixel across the display.
An examplary circuit for compensating an active-matrix OLED display is found in reference to U.S. Pat. No. 6,414,661, entitled, “Method and Apparatus for Calibrating Display Devices and Automatically Compensating for Loss in their Efficiency Over Time.” The '661 patent describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual OLED in an OLED display device, calculates and predicts the decay in light output efficiency of each pixel, based on the accumulated drive current applied to the pixel, and derives a correction coefficient that is applied to the next drive current for each pixel. The present invention further provides a method for calibrating a display device formed of an array of individually adjustable discrete light-emitting devices (pixels) by use of a camera that has an array of radiation sensors or a single photodetector.
While the '661 patent describes a suitable method of providing compensation, it does so by using a complex process of capturing images of each pixel with a camera system. A need exists for a way to provide threshold voltage compensation to overcome brightness non-uniformity without a complex system.
One objective of the invention to provide an active-matrix OLED display that has uniform brightness from pixel-to-pixel across the full area of the display by overcoming brightness non-uniformity caused by irregularities of the manufacturing process.
Another objective of the invention to provide a simplified system for and method of compensating the brightness of a flat-panel on a pixel-by-pixel basis.